This invention relates generally to the art of electronic integrated circuits and, more particularly, to circuits and techniques for initializing bistable circuit elements when power is turned on to these elements.
Digital integrated circuits all contain bistable semiconductor elements in the form of registers, latches, flip-flops, memory elements and the like. Each of these circuit elements generally has two portions, one of which is "on" while the other is "off". When power is turned on to such a circuit element, the voltage increases from zero to its maximum value over a finite, although short, time. When the voltage reaches the threshold level of one of the portions of such a circuit element, the element will initialize into one of its two states. Since it is difficult for all elements to be formed on an integrated circuit chip with exactly the same threshold voltage, it is impossible to control in the manufacturing process the initial stage that will be assumed by each bistable circuit element when power is turned on to it.
It is, therefore, common to include with each such bistable circuit element an additional transistor placed in one portion or the other of a bistable element, depending upon which initial state is desired for the element when the power is turned on. This transistor operates in response to an initialization signal. A special monitoring element is employed to maintain the initialization signal until the monitor changes its state when the power supply voltage rises above its threshold. But since the threshold voltages among the various semiconductor circuit elements can vary as a result of their processing, it has been found that many of these elements have not settled into their initial state before the initialization signal is terminated. The result is that certain circuits will randomly carry initial states that are different from those desired.
One technique that has been utilized to overcome this problem is to add a timing circuit as part of the initialization circuit, so that the initialization signal remains for a preset period after the supply voltage exceeds the monitoring element threshold. This gives some extra time for those bistable circuit elements that might have higher threshold voltages to settle into the desired initial state before the initialization signal is terminated. But such timing circuits are difficult to control in the manufacturing process.
Therefore, it is a principal object of the present invention to provide an improved initialization circuit and technique that allows all bistable elements in an integrated circuit to be initialized in their desired states, one that is reproducible from chip to chip, and one that operates equally well under varying power supply voltage.